. >> as flags at the white house are lowered to half-staff, president obama praises those who rush to aid the injured. >> you want to know who we are? what america is? we respond to evil. that's it. selflessly, compassionately. unafraid. >> good day, i'm andrea mitchell live in washington. the fbi and intelligence agencies are involved in an intensive search for evidence and we have new details, breaking news about the explosives used in the boston bombings, joining me now, nbc news justin correspondent pete williams and michael leiter. pete, ha have you learn about the bombs themselves? >> well, what we're told by authorities, andrea, is that they're crudely made. they appear to have been assembled inside a pressure cooker. it may be some of our younger viewers don't know what that is, but in the olden days when you wanted to make something tender, you put it in this pot with a tight lid on it. and the steam wouldn't escape and you could build up pressure and things cook faster in them. the premicrowave days. inside a pot like this, a pressure cooker is assembled the device. it inclu